This invention relates to the preparation of sintered magnets.
Medical nuclear magnetic resonance computed tomographs (NMR-CT) operating in a magnetic field of 1 to 10 kG (kilogauss) have been developed to represent sectional images of a body. The NMR-CT imaging systems generally use magnetic field generating means in the form of normal conducting magnets or superconducting magnets, with permanent magnets being advantageous because of no power consumption and a weak leakage magnetic field.
One example of permanent magnet circuit is disclosed in Gluckstern et al, U.S. Pat. No. 4,538,130. Referring to FIG. 3, there is illustrated a segmented ring magnet which includes inner and outer magnet groups 2 and 3 each comprising rectangular segments 1 arranged in a ring configuration. The magnet segments 1 each having an axis of easy magnetization as shown by a solid wedge are arranged in the inner and outer magnet groups 2 and 3 so as to produce a uniform upward magnetic field within the interior of the ring. Tuning means is provided for moving at least one magnetic segment radially relative to the ring. There is established a tunable permanent magnet circuit producing a uniform transverse magnetic field.
In the permanent magnet circuit illustrated, those rectangular magnet segments oriented in directions other than the radii of X and Y directions must have an axis of easy magnetization oblique to one side of one rectangular surface thereof.
The magnet materials used in such permanent magnet circuits are preferably sintered magnets of iron-boron-rare earth metal alloys and related materials because of their maximum energy product. Sintered magnets are generally produced by molding a powder of the material under pressure into a compact and sintering the compact. When a compact having an axis of easy magnetization oriented at an angle to one side of its rectangular surface is molded, sintering of the compact deforms the rectangular surface into a parallelogram because Fe-B-rare earth metal materials exhibit a great difference in percent shrinkage upon sintering in the orientation direction and directions perpendicular thereto.
For this reason, it is not a practice to directly produce a sintered rectangular magnet block having an axis of easy magnetization oriented at an angle to one side of its rectangular surface. A common method is by preparing a sintered rectangular magnet block with an easy axis of magnetization oriented parallel to one side thereof and cutting the block along predetermined directions. This method has the disadvantages that not only cutting operations are cumbersome, but the loss of the material wasted reaches 40 to 150% based on the completed product, resulting in an increased cost of material.
Gluckstern et al propose in the above-incorporated patent a method comprising providing a sintered rectangular magnet with an easy axis of magnetization oriented parallel to one side thereof, cutting the magnet along predetermined directions into four elements, and reassembling the elements to form a new rectangular magnet having an axis of easy magnetization of a desired orientation. This method is inconvenient in delicate cutting of sintered magnet and reassembling.